1. Field of the Invention
The present invention relates to a honeycomb structure made of cordierite used as a catalyst carrier, in a device for cleaning exhaust gases from an internal combustion engine, and to a method of producing the same.
2. Description of the Related Art
As a catalyst carrier for a device for cleaning exhaust gases, as shown in FIGS. 1 and 2, there has heretofore been used a honeycomb structure 9 in which partitioning walls 90 made of cordierite are arranged in the form of a honeycomb to form a number of cells 99. A catalyst 8 for cleaning exhaust gases is carried on the surfaces of the partitioning walls 90 of the honeycomb structure 9 to exhibit the function of cleaning exhaust gases.
With respect to the honeycomb structure in recent years, as stringent regulations are enforced against exhaust gases from the automobiles, it has been urged to activate the catalyst of the device for cleaning exhaust gases as quickly as possible. To cope with this, there has been proposed a method of lowering the heat capacity of the honeycomb structure as a whole to shorten the time period for elevating the temperature. Concretely, the thickness of the partitioning walls in the honeycomb structure is decreased, to decrease the weight of the catalyst carrier, in order to lower the heat capacity.
However, a limitation is imposed on decreasing the weight by decreasing the thickness of the partitioning walls due to problems from the standpoint of production. It has therefore been desired to develop means for lowering the heat capacity of the honeycomb structure as a whole relying upon a method other than the method of decreasing the thickness of the partitioning walls.
The catalyst is carried by the partitioning walls of the honeycomb structure by coating the partitioning walls with a slurry containing the catalyst, followed by drying. In the conventional honeycomb structure, the partitioning walls have a low water-absorbance, and the amount of the slurry that can be applied at one time is small. Therefore, the application and drying are repeated slurry several times, or a slurry containing the catalyst at a high concentration must be applied.
The application of several times greatly decreases a production efficiency. When the slurry having a high concentration is applied, on the other hand, the thickness t.sub.2 of the catalyst carried at the corners of the cells 99 become very large; losing balance relative to the thickness t.sub.1 of the catalyst carried on general portions, as shown in FIG. 3. Concretely, it is desired that the surface of the catalyst layer is located at position of a broken line 71 in FIG. 3, but the thickness of the catalyst layer increases up to the position of a solid line 72.
In this case, the surface areas decrease in the cells, and the reaction-promoting effect of the catalyst decreases. Also, this increases the flow resistance of gases passing through the cells. In other words, the efficiency of the catalytic reaction decreases.
Therefore, it has heretofore been desired to develop a honeycomb structure having partitioning walls exhibiting water-absorbability larger than that of the prior art and capable of being coated with a slurry having a suitable degree of concentration in an amount larger than that of the prior art.
The present invention was accomplished in view of the problems inherent in the above prior art, and provides a honeycomb structure having partitioning walls exhibiting excellent water-absorbability and is capable of activating the carried catalyst within a short period of time, and a method of producing the same.